The present invention generally relates to a heterojunction bipolar transistor and a method of producing the same. More particularly, the present invention is directed to a narrow band gap base type heterojunction bipolar transistor and a method of producing the same.
Recently, heterojunction bipolar transistors of two different types have been proposed, one of which is a wide band gap emitter type heterojunction bipolar transistor, and the other of which is a narrow band gap base type heterojunction bipolar transistor. In the wide band gap emitter type of heterojunction bipolar transistor, the emitter is formed of a material having a band gap wider than that of silicon. In the narrow band gap base type of heterojunction bipolar transistor, the base is formed of a material having a band gap narrower than that of silicon, such as silicon-germanium (SiGe). The present invention is directed to the latter type. Hereinafter, a heterojunction bipolar transistor is simply referred to as an HBT for the sake of simplicity.
FIG. 1 is a cross sectional view of a conventional narrow band gap base type HBT having a SiGe base. The illustrated HBT includes an n-type silicon collector layer 1, a p-type silicon-germanium base layer 2, an n-type silicon emitter layer 3, an insulating layer 4 formed of silicon oxide (SiO.sub.2), a base electrode 2B and an emitter electrode 3E. A collector electrode to be formed on the collector layer 1 is omitted for convenience' sake. The base layer 2 is grown on the collector layer 1 by a molecular beam epitaxy (MBE) process or a rapid thermal epitaxy process. Then the emitter layer 3 is grown on the base layer 2 in the same process as the base layer 2. A heterojunction is formed at an interface between the base layer 2 and the emitter layer 3. Since the base layer 2 is formed of SiGe, the band gap of the base layer 2 is narrower than that of the collector layer 1 formed of silicon, so that a high current transfer ratio (h.sub.FE) can be obtained. Additionally, an impurity can be contained in the base layer 2 at a high concentration so that the base resistance can be reduced and higher operational speed can be obtained. In addition to the above-mentioned reasons, the fact that the n-type emitter layer 3 does not contain a different type impurity (p-type impurity) contributes to an increase of the current transfer ratio (h.sub.FE) and makes it possible to operate at low temperatures.
In the layer structure shown in FIG. 1, the emitter layer 3 is electrically coupled to the base electrode 2B through a long length of the base layer 2. In addition, conventionally, the base layer 2 is as thin as 1000 angstroms. For these reasons, the base resistance is large, which prevents the HBT from operating at high speeds.
It is noted that Japanese Laid-Open Patent Application No. 63-138773 discloses a wide band gap emitter type HBT and teaches that the base electrode is positioned as close to the base-emitter junction as possible in order to reduce the base resistance. The proposed layer structure is effective in the wide band gap emitter type. However, there is room for improvement in reduction of base resistance. Additionally, a process of producing the proposed layer structure is complex.